The circuitry of an integrated circuit (IC) chip (e.g., a microprocessor, a random access memory, a microcontroller, an application specific integrated circuit, and others) is typically connected to another circuitry through interconnect structures, such as interposer, substrate, and board. An interposer provides electrical interconnections between an IC and its package; a substrate typically provides connection between an IC chip and a board; a board typically provides connections between various components. To make the electronic equipments smaller, faster, lighter and less expensive, high-density interconnect structures are fabricated to accommodate a large number of wires per unit area in substrates or boards. For example, using a sequential build up approach, a printed circuit board (or printed wiring board) can be fabricated through adding layers of circuitry over layers of dielectrics and through selectively removing the dielectrics to form microvia connections and plated through hole connections among layers of circuitry that are separated by the dielectrics. In a high-density interconnect structure, wires in a package can be routed through multiple layers of traces and through the microvias between the layers of traces. Traditional laminates, such as flame retardant woven glass reinforced epoxy resin or bismaleimide-triazine epoxy resin, have been used for chip-scale packages designed with various mother board interfacing styles, such as pin-grid array (PGA), ball-grid array (BGA) and land grid array (LGA).
Multichip Module (MCM) is a packaging approach based on the interconnection of multiple bare die on a single substrate. Polyimides have been used as dielectrics in MCM types of structures, in which polyimides are spin coated to form the dielectric layers. Sodium or potassium hydroxide (NaOH/KOH) treatment of polyimide has been used to promote copper adhesion to polyimide.
Conductive traces are typically formed through a metallization process. In a subtractive metallization process, a blanket layer of metal is formed on the substrate; and a photoresist and metal etch process is used to remove a portion of the blanket layer to define the traces. In a fully-additive metallization process, metal traces are directly formed on the substrate. In a semi-additive metallization process, a blanket seed layer of metal is formed on the substrate; after the traces of a desired thickness are formed using a photoresist and plating process, the seed layer outside the traces is removed. A metallization process can be performed through either electroless plating or electrolytic plating. Electroless plating uses metal deposition from solution by chemical activation of the surface. Electrolytic plating uses current-induced metal deposition through an electrolyte.
The holes for microvias can be obtained through drilling the dielectric layer. The high temperature generated during drilling can cause the melting and smearing of the epoxy-resin base material over the conductive contacts in the hole. The smear may cause a defective circuit. Desmear is a process to remove the resin smear and clean the hole. Typically, a sodium or potassium permanganate solution is used to desmear. The permanganate desmear may include epoxy sensitizing, permanganate etch, and neutralizing. The sensitizer swells the epoxy and facilitates the subsequent removal. The permanganate solution etches the epoxy by oxidizing the covalent bonds within the polymer network. The neutralizing bath removes permanganate from the oxidized hole and panel surface.